The coroner said Elisa Lam, 21, climbed into the water tank.
The autopsy listed Lam's bipolar disorder under other findings.
Authorities say Lam's body was found at the Cecil Hotel Feb. 19 when a maintenance worker was checking the water tank. There had been complaints about a drop in water pressure.
Lam had been missing since around Jan. 31. She arrived in Los Angeles from British Columbia on Jan. 26. The nature of her visit was unclear.